1. Field of the Invention
This invention relates to the field of antennas, and more particularly to the field of wideband antennas.
2. Introduction
There is an increasing interest in wideband, low noise feeds for the next generation radio telescopes. Ultra wideband feeds are essential for sweeping over large frequency ranges, frequency agility, detection of short duration pulses, multi-frequency imaging, and simultaneous observation of several spectral lines.
Traditionally, radio telescopes make use of feed horns for illuminating the parabolic aperture because of their simplicity, ease of excitation, versatility, large gain, and preferred overall performance. Feed horn bandwidths are limited to less than an octave and, hence, typically a set of feed horns operating at different frequencies is used to observe over a wideband range. A feed for parabola is situated such that its phase center coincides with the focus of the parabola. Different frequency bands can be selected by changing the feed horns. In some cases, it is important to study a scientific phenomenon by observing a source simultaneously at different frequencies. Because of the mechanical movement involved, it is not possible to achieve instantaneous wide bandwidth with traditional feed horns. Hence there is a need for a feed which will allow simultaneous multi frequency observations.
In the past, wide bandwidth feeds have been developed using log periodic structures of line resonators or elements stacked in the form of pyramids such as e.g. zig-zag elements used in the Allen Telescope Array or the trapezoidal geometry used in the Green Bank Solar Radio Burst Spectrometer (GB/SRBS). Other applications include, radar, measurement range, ultra wideband radio, and spread spectrum communications. Such a feed, however, has a phase center that varies with frequency. The Eleven Antenna developed by the Chalmers Group solves the varying phase center problem. The commercially available open boundary quadridge horn and quasi self-complementary (QSC) feed being developed at Cornell are other examples of decade bandwidth feeds.
A wideband, fixed phase center, dual polarized, low loss feed with an integrated low noise amplifier (LNA) was developed. The far field patterns of the feed-LNA integrated unit were measured including E, H, Co- and Cross-polarization over 0.5-4 GHz frequency range. The beamwidth was nearly constant and the phase center remained close to the center of the ground plane over the entire frequency range. However, it was found that overall the antenna lost the self-complementary nature in the presence of the ground plane, and hence, led to frequency dependent impedance variations.